Marine snow and epipelagic suspensoids in the Reda carbonates and a pronounced mid-Ludfordian (Silurian) CIE in the axis of the Baltic Basin (Poland)

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Marine snow and epipelagic suspensoids in the Reda carbonates and a pronounced mid-Ludfordian (Silurian)

CIE in the axis of the Baltic Basin (Poland)

WOJCIECH KOZŁOWSKI Faculty of Geology, University of Warsaw,

Żwirki i Wigury 93, PL-02-089 Warsaw, Poland. E-mail: woko@uw.edu.pl ABSTRACT:

Kozłowski, W. 2020. Marine snow and epipelagic suspensoids in the Reda carbonates and a pronounced mid-Ludfordian (Silurian) CIE in the axis of the Baltic Basin (Poland). Acta Geologica Polonica, 70 (4), 529–567. Warszawa.

The mid-Ludfordian pronounced, positive carbon isotope excursion (CIE), coincident with the Lau/kozlowskii extinction event, has been widely studied so far in shallow-water, carbonate successions, whereas its deep-water record remains insufficiently known. The aim of this research is to reconstruct the sedimentary environments and the palaeoredox conditions in the axial part of the Baltic-Podolian Basin during the event. For these pur- poses, the Pasłęk IG-1 core section has been examined using microfacies analysis, framboid pyrite diameter and carbon isotope measurements. The prelude to the event records an increased influx of detrital dolomite interpreted as eolian dust, coupled with a pronounced decrease in the diameter of the pyrite framboids, indicating persistent euxinic conditions across the event. The event climax is recorded as the Reda Member and consists of calcisiltites, composed of calcite microcrystals (‘sparoids’), which are interpreted as suspensoids induced by phytoplankton blooms in the hipersaturation conditions present in the epipelagic layer of the basin. Both the prelude and climax facies show lamination, interpreted as having resulted from periodical settling of marine snow, combined with hydraulic sorting within a ‘benthic flocculent layer’, which additionally may be respon- sible for a low organic matter preservation rate due to methanogenic decomposition. Contrary to the observed basinward CIE decline in the benthic carbonates in the basin, the Reda Member records an extremely positive CIE (up to 8.25‰). Given the pelagic origin of the sparoids, the CIE seems to record surface-water carbon isotope ratios. This points to a large carbon isotope gradient and kinetic fractionation between surface and bot- tom waters during the mid-Ludfordian event in a strongly stratified basin. The Reda facies-isotope anomaly is regarded as undoubtedly globally triggered, but amplified by the stratified and euxinic conditions in the partly isolated, Baltic-Podolian basin. Hence, the common interpretation of the basin record as representative for the global ocean needs to be treated with great caution.

Key words: Pelagic carbonates; Sparoids; Marine snow; Algal blooms; Eolian dust; Carbonate supersaturation; Euxinia; Baltic Basin; Reda Member, Ludlow; Silurian.

INTRODUCTION

The mid-Ludfordian contains a globally recorded, facies-geochemical event, coinciding with the Lau conodont (Jeppsson and Aldridge 2000) and kozlow-

skii graptolite (Koren 1993, Urbanek 1993) extinc- tions (summarized in Calner 2008; Jeppsson et al.

2012; Munnecke et al. 2010). One of the most con- spicuous features of this event is the pronounced positive mid-Ludfordian carbon isotope excursion (CIE),

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globally recorded in the carbonate successions of low and middle palaeolatitudes (e.g. Frýda and Manda 2013; Jeppsson et al. 2012; Spiridonov et al. 2017;

Younes et al. 2017 and references therein). The sed- imentary record of the event closely resembles other CIE-related early Palaeozoic perturbations (sum- marized in fig. 6 in Munnecke et al. 2003), which probably represent the repercussions of short-lived glaciations in low-palaeolatitudes (see discussion in Cherns and Wheeley 2009; Kaljo et al. 1996; Kaljo et al. 2003). According to Munnecke et al. (2003), typi- cal sedimentary features of these CIE-related events observed in shallow-water tropical shelf successions include: (1) proliferation of carbonate platforms, particularly through the intensification of microbial-bio- chemical and (quasi-) chemogenic carbonate precipitation; (2) a sequence boundary near the base of the event; (3) abundant terrigenous silt admixture, often interpreted as being of eolian origin (e.g. Kozłowski and Sobień 2012; Samtleben et al. 2000); (4) coin- cident low diversity of plankton communities and minor extinctions; and (5) short duration (low relative thickness). Munnecke et al. (2003) have noticed also that the events are not coeval with the deposition of anoxic sediments sensu organic matter (OM) ac- cumulations in deep shelf environments, which, in turn, is observed below and above the event interval.

Loydell (2007) complemented the list by basinward decline of the CIE amplitude; however, CIEs with a minor amplitude are recorded in organic matter (OM) carriers also in deep shelf (Noble et al. 2012) and oceanic settings (Melchin and Holmden 2006a;

Underwood et al. 1997). The maximum amplitude of CIEs, especially during the mid-Ludfordian CIE, which is extreme in amplitude (up to +12), seems to be locally amplified by isotope fractionations in carbonate-bearing epeiric seas with limited circulation (Holmden et al. 2012; Kozłowski 2015; Kozłowski and Sobień 2012). Despite a similar scenario of the early Palaeozoic events and a general consensus on their glacial trigger, the nature and origin of CIEs is still the subject of debate (summarized and discussed in Calner 2008; Ghienne et al. 2014; Kozłowski 2015; Loydell 2007; Melchin and Holmden 2006a;

Munnecke et al. 2010; Munnecke et al. 2003; Noble et al. 2012).

The late Silurian, Caledonian (Tornquist branch) foreland of the East European Craton, extending from Scania in Sweden to Podolia in the Ukraine (Central Europe; Text-fig. 1), is one of the type areas of the Lau/kozlowskii event, intensively studied in re- cent years (Eriksson and Calner 2008; Jeppsson et al.

2012; Jeppsson et al. 2007; Kaljo et al. 2015; Kaljo et

al. 2007; Kozłowski 2015; Kozłowski and Munnecke 2010; Kozłowski and Sobień 2012; Martma et al.

2010; Spiridonov et al. 2017; Younes et al. 2017).

The record of the Lau/kozlowskii event in the Baltic- Podolian Basin is well recognised in its littoral part with an extended carbonate platform. In the most proximal parts, sea-level fall caused a hiatus due to emersion (Eriksson and Calner 2008; Kaljo et al.

2015; Kaljo et al. 1997; Kozłowski and Munnecke 2010), whereas facies shift resulted in extensive car- bonate (e.g. Martma et al. 2010) and/or clastic-car- bonate (Eriksson and Calner 2008; Kozłowski and Munnecke 2010) shallow-water sedimentation in a more distal ramp setting (Text-fig. 1).

The record of the event in deep-water settings of the basin is poorly recognized. In the offshore pa- laeobasin in the subsurface area of Poland, the event interval is marked by a distinct negative, natural gamma-ray anomaly (Topulos 1976; Topulos 1977), used by Tomczyk (e.g. 1970) for tracking the ‘middle Siedlce’ horizon, referred to the Neocucullograptus–

Formosograptus graptolite interregnum (with the base coeval with the kozlowskii event of (Urbanek 1993). Kozłowski and Sobień (2012) have shown that the ‘middle Siedlce’ geophysical marker in a periplatform setting (Mielnik IG-1 borehole section, no.

14 in Text-fig. 1), corresponds to a distinct facies anomaly, coeval with the mid-Ludfordian CIE. The anomaly is expressed by a mass admixture of calcite, dolomite and quartz silt forming a peculiar limestone-marl horizon, referred in northern Poland to the Reda Member (Modliński et al. 2006; Porębski and Podhalańska 2019).

The Reda Member in the periplatform setting of the Mielnik IG-1 borehole section contains abundant detrital silt (quartz, dolomite), which was interpreted as eolian-derived (Kozłowski 2015; Kozłowski and Sobień 2012). The mass appearance of minute euhedral calcite crystals was taken as massive carbonate precipitation in the water-column (whitings) caused by carbonate hypersaturation conditions (Kozłowski 2015). The anomalous facies in the Mielnik IG-1 borehole section records a moderate-high CIE amplitude in bulk rock samples (+6.74‰), resulting from the mixing of epipelagic calcite with δ¹³C values ~10‰, suppressed by admixture of eolian dolomite with low (~0‰) δ¹³C values (Kozłowski 2015). Despite the lack of OM accumulation, synchronous pronounced changes in the diameter of pyrite framboids in the carbonate horizon indicate the development of euxinic conditions in the water column (Kozłowski 2015), which was confirmed later by geochemical proxies from other parts of the basin (Bowman et al. 2019).

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Text-fig. 1. Location of the Pasłęk IG-1 borehole section (arrowed) against Silurian palaeogeography and facies distribution (based on Bassett et al. 1989) during the mid-Ludfordian regression on the SW margin of the East European Craton. The isopachs (red lines) illustrate the thick- ness of the rocks occurring between the mid-Homerian (Mulde) and mid-Ludfordian (Lau) reference geophysical anomalies. Selected Silurian sections: 1 – Ohessaare-1; 2 – Ventspils-D3; 3 – Pavilosta-1; 4 – Priekule-1; 5 – Gotland; 6 – Skåne; 7 – Vidukle-61; 8 – Milaičiai-103; 9 – Vilkaviškis-134; 10 – Gołdap IG-1; 11 – Bartoszyce IG-1; 12 – Pasłęk IG-1; 13 – Lębork IG-1; 14 – Mielnik IG-1; 15 – Rzepin (Holy Cross

Mts.); 16 – Syczyn OU-1; 17 – Kotiuzhyny-1; 18 – Zvanets (Podolia).

100 200

50 100

200 150

100 1000

2000

1500

250

300

150 20

0 300

30 0 500

50 0 40 0

40 0 100

0

50

0

500

Poland

Belarus

Lithuania Latvia

Estonia

BALTIC SEA Sweden

Ukraine Slovakia

Romania

N

1

2 3 4 6 5

8 7

10 9 11 13

12

14 15

16

17

18

Baltic Basin

Pasłęk IG-1

Mid-Ludfordian basin belts:

tectonic deformations:

emergent carbonate shelf reefs & shoals

outer carbonate ramp periplatform offshore basin (pelagic zone) foredeep submarine fan clastic wedge top -submarine clastic wedge top -emergent

Variscan Caledonian

100

1

Mulde-Lau thickness in meters isopach

erosional limit of the Silurian tectonic shortening

selected profiles state borders

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The Reda Member gamma-ray anomaly seems to be palaeobathymetrically independent and may be tracked basinward, to the clay-dominated, basinal facies zone (Kozłowski and Sobień 2012; Porębski and Podhalańska 2019; Porębski et al. 2013; Topulos 1977). These observations suggest the pan-basinal extent of the massive epipelagic calcite precipitation and eolian dolomite admixture, extending also in the distal and strictly pelagic realm.

Based on this statement, the aim of this study is to investigate the facies-geochemical record of the Reda anomaly (Lau/kozlowskii event) in the maxi- mally offshore – deep water part of the Baltic Basin, particularly in terms of: (I) comparing the facies development between the periplatform (Mielnik IG-1 borehole section) and strictly pelagic settings; (II) reconstructing the sedimentary conditions and pro- cesses leading to the formation of unusual pelagic carbonates; (III) understanding the time relation between the facies anomaly and the development of euxinic conditions in the deepest part of the basin; and (IV) determining the amplitude of the carbon isotope excursion in context of its lateral and paleobathy- metric variation. For these aims, the Reda Member succession from the basin axis, represented by the Pasłęk IG-1 borehole section in northern Poland (Text-fig. 1), has been studied in terms of sedimentol- ogy (microfacies), the carbon and oxygen stable isotope record and the record of redox conditions based on the distribution of pyrite framboid diameters.

GEOLOGICAL AND PALAEOGEOGRAPHIC SETTING

The Ediacaran to lower Palaeozoic succession of NE Poland forms the lower part of the sedimentary cover of the East European Craton, which comprised the Baltica sector of the Laurussia palaeocontinent in the middle Palaeozoic. During the late Silurian, the area was located in the tropics (Cocks and Torsvik 2005), which resulted in the presence of the carbonate- bearing successions of palaeolittoral areas. The belt of shallow-water carbonate platforms can be traced from Gotland island (Sweden) in the north, to Besarabia (Romania and Moldova) in the south (Text-fig. 1).

The more distal part of the craton margin, extending from NW to SE across central Poland, evolved during the Silurian from an epicratonic marginal shelf basin into the foreland basin of the southern branch of the Caledonian orogen (Poprawa et al. 1999), as a con- sequence of oblique arc- continent collision with the Teisseyre Arc (see e.g. Kozłowski et al. 2014).

During the Ludlow, massive clastic influx from the newly formed Tornquist branch of the Caledonian orogen overfilled the proximal (SW) part of the foredeep (Jaworowski 1971), creating a novel littoral zone (n. 15 in Text-fig. 1; Kozłowski 2003) located opposite the main (Gotland-Estonia-Podolian) carbonate shelf. As a result of such palaeogeographic changes, the earlier open shelf basin was transformed into elongated epicratonic seaway. Because the Baltic Basin is the only northernmost, erosive remnant of such a paleobasin (see Text-fig. 1), it is proposed here to name it as the Baltic-Podolian (or Paratornquist analogously as Paratethys) Sea (or Basin).

It is important to note that the continuous occur- rence of an open marine fauna across the Lau event in the basin (e.g. Martma et al. 2010; Spiridonov et al. 2017; Urbanek 1997) indicates persistent normal- marine conditions during the mid-Ludfordian low- stand, despite palaeogeographical restrictions. The most probable connection with the ocean could have been located in the far south-east, where still exis- tent plate divergence is recorded by the numerous pyroclastic beds in the Ludlow-Přídolí succession of Podolia (Huff et al. 2000).

The studied section of the Pasłęk IG-1 deep borehole (Text-fig. 2A) is located in the Peribaltic Syneclise (southern Baltic Basin) in northern Poland (n. 12 in Text-fig. 1, arrowed). The section was selected for this study following a palaeogeographic, facies and thickness pattern analysis. The pattern of changes in thickness between the Mulde and Lau event-associated natural negative gamma anomalies (isopach in Text- fig. 1), indicates that the studied section represents the most distal part of the foredeep clastic wedge, hence it is representative for the basin axis facies zone. The event-related carbonate marker of the Reda Member occurs within the Kociewie Formation (Text-fig. 2A), recognised as comprising flysch-like sedimentary rocks, deposited with the participation of turbidity currents (Jaworowski 1971; Jaworowski 2000) in a distal foredeep setting. Subsequent elements of the Silurian succession of the Pasłęk IG-1 borehole section (Tomczyk 1973) are in general typical for the entire SW margin of the East European Craton.

The stratigraphically incomplete Llandovery succession (see Text-fig. 2A) is represented by an only 20 m thick complex of black siliceous shales (Pasłęk Formation), succeeded by 230 m of dark-grey clayey graptolitic shales (Pelplin Formation) representing the Wenlock and lower Ludlow. The general lack of benthic fauna, abundant graptolites and scarcity of primary carbonates are consistent with its pelagic sedimentary setting and basinal palaeogeographic

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PERMIANS I LU R IAN

ORDOVICIAN

SHEINWOODHOMERIANGORSTIANLU D F O R D IAN

Lower SiedlceMielnikUpper SiedlceMiddle S.

2638 2618

C.lundgreni G.nassa C. ludensis N.nilssoni L. scanicus + + S.chimaera L. progenitor S. leintwardinensis B. tenuis B. praecornutus B. cornutus N. inexpectatus

N. kozlowskii Ps. latilobus

Pr. dubius postfrequens

interzone S. balticus U. acer U. aculeatus

U. spineus no graptolites

2560 25252518 2500 2471 2460 2425 2400 2386 2345 2300 2200.5 2156 2133 2122 2055 2010 1966,5

Reda MemberKociewie FormationPelplin Formation

Pasłęk Formation

Puck FormationLithostratigraphy (after Modlinski et al. 2006)

Lithological column (Tomczyk 1973, upgraded)

Graptolite Biostratigraphy (Tomczyk 1973;

for upper Ludfordian - this paper) Local

Global Chrono- -stratigraphy

1990 1995 2000 2005 2010 2015 2020 2025 2030 2035 2040 2045 2050 2055 2060 2065 2070 2075 2080 2085 2090 2095 2100 2105 2110 2115 2120 2125 2130 2135 2140 2145 2150 2155 2160 2165 2170 2175 2180 2185 2190 2195 2200 2205 2210 2215 2220 2225 2230 2235 2240 2245 2250 2255 2260 2265 2270

20 40 60 80

Bohemograptus bohemicus tenuisP. dubius - - L. posthumusno graptolitesPseudomonoclimacis latilobus Slovinograptus sp. Slovinograptus balticus Uncinatograptus acer U. acer aculeatus Neocolonograptussp.

U. gr. spineus

Monograptus lebanensis Formosograptus formosus

¹³C ‰ PDB

A B

1982

2123

2186 2166

2386

Łyna Member

Facies contribution [%]

-2 0 +2 +4 +6 +8

Explanation for lithology column:

calcareous shales siltstones

mudstones

shales

red silt laminae marls

laminated calcisiltite massive calcisiltite limestones

claystones

depth [metres]

Reda Member

C D

siltstone (P)

fine claystone (J)

limestone (W) mudstone (K)

laminated claystone (L) silt laminated mudstone (O) silty mudstone (N) calcareous/dolomitic heterolite (R)

massive calcisiltite (D)

dolomitic mudstone (M)

laminated calcareous mudstone (C)

laminated marly calcisiltite (B)

laminated calcisiltite (A)

Explanation for facies contribution column

LU D LO W IANW E N LO C K IAN

LLANDOVERIAN

Text-fig. 2. A – Vertical column (scaled to depth) of Silurian strata in the Pasłęk IG-1 borehole; B – Detailed vertical changes in the facies contribution of the upper Ludfordian in the Pasłęk IG-1 borehole; C – Graptolite ranges in the section; D – General bulk carbonate carbon

isotope curve (this paper).

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position. The main pronounced facies anomaly within this interval occurs at the top of the Cyrtograptus lundgreni Zone (2525 m, Text-fig. 2A) with the ap- pearance of a rich non-graptolitic fauna, carbonate admixture and pronounced, natural gamma-ray low.

The anomaly, coeval with the Mulde/lundgreni event (Porębska et al. 2004) in the neighbouring Prabuty IG-1 and Olsztyn IG-2 borehole sections (incomplete core interval in the Pasłęk IG-1 borehole), was recognised (Zenkner and Kozłowski 2017) as an older facies analogue of the Reda Member (named Łyna Member; Text-fig. 2A).

The lower Silurian graptolitic shales are followed (above the Saetograptus leintwardinensis Zone – Tomczyk 1973) by the flysch-like Kociewie For- ma tion, early to middle Ludfordian in age. The formation in the Pasłęk IG-1 borehole section is dominated by the presence of fine-grained components of the Bouma sequence (F3–F4 facies of Jaworowski 2000), and it is interpreted as being deposited in a deep- water plain, influenced by distal turbidites in a foreland setting (Poprawa et al. 1999). The rela- tively lowest (in observed range) thickness for the Kociewie Formation in the Pasłęk IG-1 borehole section (270 m) points to the basin-axis position of the profile in the general palaeobathymetric and subsi- dence pattern (Text-fig. 1).

The uppermost part of the Kociewie Formation in the Pasłęk IG-1 borehole section is interrupted by the relatively thin (20 m) carbonate-bearing Reda Member, consisting of laminated marlstones and calcisiltites (Modliński et al. 2006; Text-fig. 2A).

Originally, Tomczyk (1970) referred this interval to the “Middle Siedlce Beds”, defined by him in Poland as beds containing a low-diversity pristiograptid graptolite fauna, occurring above the last appearance of the Bohemograptus–Neocucullograptus assem- blage (kozlowskii extinction event of Urbanek 1993) and below the flourishing of the Formosograptus graptolite fauna assemblages.

The Reda Member is succeeded by the topmost part of the Kociewie Formation and the clay dominated Puck Formation, containing an upper Ludfordian graptolite assemblage. Post-Silurian and pre-Permian erosion resulted in the lack of Přídolí

deposits in the studied section. The basinal facies of the Přídolí occur further northwards towards the Baltic Sea area, where development of an open marine basin is documented up to the Silurian–Devonian boundary.

MATERIAL AND METHODS

Almost 100% of the Pasłęk IG-1 borehole was cored (see Text-fig. 3B). The archival material in the studied interval is almost complete. Section measur- ing was performed in the drill core storage. The macrofacies determination was carried out by comparison with a reference sample palette, prepared from samples representative for the macrofacies spec- trum observed during a preliminary core overview.

The macrofacies were determined in constant span points, with a resolution of 0.1 m (0.05 m in the Reda Member interval). The codified macrofacies record was subsequently processed in Excel software and exported as graphic logs (Text-fig. 3A, B). Graptolites observed during the analysis were photographed and used for updating of the existing archival biostratigraphic data (Tomczyk 1973; Text-figs 2A, C; 3A).

Microfacies analysis was performed on a set of 90 uncovered, finely polished, and additionally thinned (10–20 µm) thin sections, stained (etched) in Dickson’s solution. The solution produces a pink to red stain on the calcite surface, with saturation and hue depending on crystal size, orientation and presence of chemical impurities (especially iron admixture). The dolomite remains unstained. The observations were performed in transmitted, as well as combined 40% transmitted and 60% reflected light in NikonEclipse LV100POL with 2×-60× Plan Fluor lens and LUCIA software with parallel exam- ination of thin sections and raw rock fragments in an SEM-backscatter with EDS in the Microanalysis Laboratory of the Faculty of Geology, University of Warsaw. The semi-quantitative mineral composition of 58 bulk rock samples across the studied interval was determined using XRD analysis (standard Bragg-Brentano method) in the XRD Laboratory of the Faculty of Geology, University of Warsaw.

Text-fig. 3. Detailed logs of the mid-Ludfordian CIE interval in the Pasłęk IG-1 borehole section: A – graptolite ranges; B – lithological log;

scales: depth in metres; succeeding core intervals with archival box numbers (from-to); rectangles – archival boxes, red colour – lack of core;

C – bulk carbonate carbon isotope data, colours of circles indicate facies affiliation of the analysed sample; D – bulk carbonate oxygen isotope data recalculated according to Rosenbaum and Sheppard (1986) for dolomite admixture (using semi-quantitative XRD data); E – box-and- whisker plots of pyrite framboid sizes, 5 and 10 μm size limits marked by black vertical lines; red solid lines showing the pyrite content [‰];

F – natural gamma log; G – magnetic susceptibility log; H – vertical changes in the mineralogical composition (semi quantitative XRD data) of the samples subdivided into “pelagic” (left scale) and “detrital” (right scale) components, dotted black line shows vertical changes in the

contribution of dolomite in the terrigenous silt [dolomite/(dol + Q + F ratio)].

→

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dolomitic mudstone (M) massive calcisiltite (D)

laminated calcareous mudstone (C) laminated marly calcisiltite (B) laminated calcisiltite (A)

fine claystone (J) mudstone (K)

laminated claystone (L) silt laminated mudstone (O) silty mudstone (N)

siltstone (P)

calcareous/dolomitic heterolite (R) Linograptus posthumus Pristiograptus dubius sl. Pseudomonoclimacis latilobus

Slovinograptus sp. Monograptus lebanensis

0 ¹³+2C ‰ PDB+4 +6 +8

0 +2

+4 +6

+8

calcite chlorite illite dolomite quartz feldspare

Fe-dolomite

90 100 110

GR [% of average dose]

80

100 150 200 250 MS*10^-6SI10g 50

2 3 4 5 6 7 8 9 10

pyrite content [‰]¹²³⁴⁵⁶⁷⁸⁹ median

mean 1st quartile

3rd quartile

min. max.

Lithology explanation (macrofacies code):

H G

F E

D

Reda Member

-10¹⁸O ‰ PDB-8 -6

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Statistical counting and measurement of pyrite framboids were performed on the scaled SEM- backscatter images of thin sections using LUCIA software, with a precision greater than 0.1 μm. The measurements and counting were performed only for well-preserved, well-shaped and non-aggregate framboids. Euhedral, shapeless or aggregated pyrites (all categories rarely observed), were rejected from the measurement procedure. The diameters of framboidal pyrites were measured along serial parallel scanning sectors (150 μm width), perpendicular to lamination, across each thin-section until a size population of more than 300 framboids was achieved, or to the end of the measurement sector (resulting in >300 populations). The measured apparent diameters of pyrite framboids are smaller than the true values, but the deviation is less than 10% (Wilkin et al. 1996; Wignall and Newton 1998). The minimal,

mean and maximum diameter; the standard deviation of diameters, the percentage of pyrite framboids with diameter ≥10 μm and pyrite content [‰] in samples were calculated from the measured framboidal pyrite population in each sample and the known size of the investigated area. The analytical results are listed in Table 1. The data have been presented (Text-fig. 3E) in the form of standard box and whisker plots proposed by Wignall and Newton (1998).

δ¹³C and δ¹⁸O values were measured in 107 bulk rock samples in the Stable Isotope Laboratory of the Polish Academy of Sciences in Warsaw. Each pow- dered sample was treated with 104% orthophosphoric acid at 70°C in a Thermo Kiel IV preparation system and analysed in a conjunct Thermo-Finnigan Delta Plus mass spectrometer in a Dual Inlet system. The values are reported in the conventional delta notation in per mil with respect to the Vienna Pee Dee Belemnite

sample depth

examined area (sq. mm) N

pyrite content

[%] min max mean 1Q 3Q

median

(2Q) SD

skew- -ness

grains under 5 m

[%] interpreted paleoredox conditions pyrite

type

Ps1107g 2140.3 1.28 294 1.14 2.29 31.45 7.96 5.35 6.66 9.01 4.28 1.99 21 D upper dysoxic Ps1115s 2148.5 1.00 340 3.08 2.17 71.99 10.72 5.06 8.34 14.60 8.19 2.67 25 D upper dysoxic Ps1120d 2153.7 1.14 309 0.53 2.23 24.12 5.00 3.68 4.41 5.55 2.29 3.47 67 B lower dysoxic Ps1122d 2155.4 0.85 311 1.06 1.77 23.99 6.10 4.13 5.14 7.07 3.32 2.18 45 C middle dysoxic Ps1126d 2159.7 0.85 301 0.96 1.68 25.44 5.89 4.24 5.16 6.79 2.95 2.52 48 C middle dysoxic Ps1135s 2167.6 1.00 303 0.85 1.67 21.70 5.96 3.65 4.66 7.13 3.82 1.89 58 C middle dysoxic Ps1137s 2168.9 1.85 326 0.28 1.45 25.15 4.50 2.86 4.01 5.47 2.46 2.98 71 B lower dysoxic Ps1141d 2174.2 0.57 301 0.59 1.45 12.36 3.78 2.77 3.58 4.50 1.36 1.33 82 A2 temporary euxinic Ps1143d 2176.1 0.85 302 0.37 1.79 9.16 3.66 2.93 3.45 4.31 1.09 1.08 89 A1 euxinic Ps1146s up 2178.4 0.71 333 0.56 1.46 12.08 3.90 2.81 3.54 4.63 1.57 1.49 81 A2 temporary eux.

Ps1146s low 2178.4 3.27 345 0.35 1.53 20.12 6.48 3.74 5.48 7.92 3.46 1.19 45 C middle dysoxic Ps1146d 2178.9 1.14 327 0.28 1.29 11.21 3.50 2.44 3.06 4.13 1.60 1.76 85 A1 euxinic Ps1148d 2180.7 1.28 305 0.18 1.26 8.13 3.14 2.40 2.90 3.61 1.12 1.40 93 A1 euxinic Ps1150d 2182.7 1.00 303 0.26 1.41 6.78 3.30 2.48 3.12 3.96 1.05 0.72 92 A1 euxinic Ps1151d 2183.7 1.14 302 0.27 1.47 9.66 3.63 2.64 3.33 4.42 1.33 1.14 83 A1 euxinic Ps1152d 2184.7 1.14 350 0.28 1.36 9.76 3.39 2.40 3.15 4.16 1.32 1.07 89 A1 euxinic Ps1153d 2185.7 0.71 356 0.58 1.47 8.81 3.83 2.93 3.57 4.51 1.24 0.90 83 A1 euxinic Ps1154_20 2186.2 0.57 305 0.61 1.57 9.97 3.79 2.77 3.56 4.51 1.32 1.33 83 A1 euxinic Ps1155_60 2187.6 0.57 326 0.57 1.47 7.37 3.55 2.70 3.43 4.19 1.17 0.84 89 A1 euxinic Ps1159_20 2191.2 1.00 323 0.42 1.41 13.25 4.08 2.97 3.83 4.84 1.50 1.50 77 A2 temporary euxinic Ps1161m 2193.5 1.00 390 0.54 1.36 11.03 4.20 3.19 4.02 5.06 1.43 0.99 73 A2 temporary euxinic Ps1169_15 2201.9 1.28 309 0.47 1.76 18.43 4.97 3.56 4.64 5.74 2.04 1.89 60 B lower dysoxic Ps1175_80 2208.5 1.71 311 0.40 1.72 19.28 5.29 3.74 4.93 6.32 2.25 2.24 51 B lower dysoxic Ps1185_20 2217.9 1.42 314 0.50 1.96 26.49 5.40 3.87 5.14 6.37 2.31 3.35 46 B lower dysoxic Ps1196g 2227.9 2.28 320 0.80 1.91 39.83 8.51 5.51 7.34 9.99 4.96 1.98 22 D upper dysoxic Ps1205s 2235.9 1.71 306 0.32 1.34 26.35 4.75 3.46 4.40 5.45 2.30 3.87 65 B lower dysoxic

Reda member

pyrite framboids diameter

laminated calcisiltites

Table 1. Size statistics of pyrite framboids in the mid-Ludfordian Reda Member and adjacent deposits of the Pasłęk IG-1 borehole section, with interpretation of the palaeoredox conditions.

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(VPDB) by assigning a δ¹³C and δ¹⁸O values +1.95‰

and -2.20‰ to NBS19. The δ¹⁸O values of samples were corrected using the phosphoric acid fractionation factor given by Rosenbaum and Sheppard (1986), pro- portionally to the dolomite/calcite ratio obtained with XRD. Reproducibility for the isotopic analysis (1σ) after 10 successive measurements of the NBS19 standard was ±0.03‰ for δ¹³C and ±0.07‰ for δ¹⁸O. The results are compiled in the Table 2.

Carbon and oxygen isotopes were examined only in bulk rock samples. Mechanical separation of the dolomite component for the isotope analysis (see Kozłowski 2015) was not possible due to its relative low content and very small grain size. For the pur- poses of estimation of the influence of mineralogical carbonate composition on the bulk carbonate isotope ratios, adjacent samples with contrasting calcite/dolomite ratios (obtained from semi-quantitative XRD)

Table 2. Bulk rock stable carbon and oxygen isotopes and semi-quantitative XRD carbonate composition of the Ludfordian rocks in the Pasłęk IG-1 borehole section.

Reda Mb. top

sample depth facies ¹³C

calcite/

/dolomite¹⁸O corrected ¹⁸O

sample

Reda Mb. base

Ps1033w 2068.3 W -7.70 -6.95 Ps1139s 2171.4 R 4.36 71/29 -10.07 -9.59

Ps1039w 2072.9 W -1.96 -6.06 Ps1139_60 2171.5 D 5.19 43/57 -8.53 -7.61

Ps1053s 2087.3 R -1.68 -9.48 Ps1140_20 2172.3 C 5.21 -7.68

Ps1056s 2090.3 R -1.80 -9.58 Ps1140d 2172.8 C 6.60 68/32 -9.19 -8.68

Ps1059g 2093.0 R -2.00 -10.14 Ps1141s 2173.7 B 7.00 -8.20

Ps1060g 2094.1 R -1.51 -9.18 Ps1142_30 2174.7 B 8.25 -7.86

Ps1062s 2096.3 R -1.65 -9.43 Ps1142d 2175.1 B 7.97 76/24 -8.25 -7.86

Ps1064s 2098.3 R -1.95 -10.25 Ps1143d 2176.1 B 8.24 85/15 -8.20 -7.96

Ps1068g 2102.1 R -1.09 -8.99 Ps1143dw 2176.2 D 8.06 -7.51

Ps1070gw 2103.9 W -6.23 -4.82 Ps1144dw 2177.0 D 8.09 83/17 -7.34 -7.07

Ps1072 2105.9 O -0.75 -8.76 Ps1144d 2177.1 A 7.43 84/16 -8.91 -8.66

Ps1076g 2110.1 R -1.66 -9.88 Ps1145sw 2177.9 D 7.80 -7.35

Ps1078g 2111.7 W -3.14 -6.26 Ps1145d 2178.1 B 7.47 82/18 -8.57 -8.28

Ps1082d 2116.7 R -1.88 -10.21 Ps1146sw 2178.9 D 7.75 -9.20

Ps1083_95 2118.4 O -0.87 -9.82 Ps1146d 2178.9 B 7.67 85/15 -10.98 -10.73

Ps1084s 2119.0 R -3.54 -7.92 Ps1147d 2179.7 B 7.54 78/22 -8.95 -8.60

Ps1085s 2120.0 R -1.75 -10.62 Ps1148d 2180.7 B 7.43 77/23 -9.16 -8.79

Ps1087g 2121.8 R -0.58 -9.39 Ps1149d 2181.7 B 5.80 76/24 -8.79 -8.39

Ps1089s 2124.0 R -0.31 -9.31 Ps1150d 2182.7 B 5.37 77/23 -9.07 -8.71

Ps1091d 2126.2 R -0.47 -9.02 Ps1150_80 2182.8 D 5.44 82/18 -8.94 -8.65

Ps1093g 2127.8 O -2.21 -6.63 Ps1151d 2183.7 A 4.62 84/16 -9.42 -9.16

Ps1095s 2130.0 R -1.52 -8.50 Ps1152_40 2184.4 B 4.38 80/20 -9.33 -9.01

Ps1096w 2131.2 W -5.06 -4.33 Ps1152d 2184.7 C 3.99 61/39 -8.55 -7.92

Ps1097_05 2131.6 P -0.69 -9.69 Ps1153_05 2185.1 C 3.11 38/62 -9.85 -8.85

Ps1097d 2132.2 R -1.18 -10.45 Ps1153_45 2185.5 M 1.33 25/75 -7.93 -6.71

Ps1099g 2133.5 R 0.05 -9.41 Ps1153d 2185.7 C 1.89 29/71 -6.91 -5.75

Ps1102s 2136.2 R 0.20 -9.19 Ps1154_20 2186.2 M 1.77 23/77 -6.98 -5.73

Ps1104s 2138.2 R -0.31 -9.37 Ps1154s 2186.5 R 1.89 50/50 -10.09 -9.28

Ps1105_20 2138.9 R 0.14 63/37 -9.64 -9.03 Ps1155_10 2187.1 M 1.00 15/85 -7.62 -6.25

Ps1106s 2139.5 R -0.78 -9.35 Ps1155s 2187.5 R 1.70 -7.25

Ps1107g 2140.3 R -0.18 -9.27 Ps1156s 2188.5 R 1.64 46/54 -9.45 -8.58

Ps1109s 2142.5 R 0.11 -9.23 Ps1157_30 2189.3 M 14/86 -1.40

Ps1111g 2144.3 R 0.55 53/47 -8.68 -7.92 Ps1157s 2189.5 R 1.53 53/47 -9.38 -8.63

Ps1111_65 2144.6 M 0.68 20/80 -7.25 -5.95 Ps1158s 2190.5 R 1.20 -9.15

Ps1113s 2146.5 R 0.42 -8.80 Ps1159_20 2191.2 M 0.71 14/86 -6.43 -5.04

Ps1115s 2148.5 R 0.32 -8.81 Ps1159_40 2191.3 M 0.97 -10.61

Ps1116da 2149.4 M 1.13 44/56 -9.37 -8.47 Ps1160_05 2192.2 R 0.94 67/33 -10.92 -10.38

Ps1116db 2149.4 D 1.54 62/38 -6.82 -6.20 Ps1160s 2192.7 R 1.28 69/31 -10.17 -9.67

Ps1117d 2150.7 R 0.16 -9.54 Ps1161_05 2193.3 R 1.20 70/30 -10.52 -10.04

Ps1119s 2152.5 O 1.64 -6.19 Ps1161d 2193.9 R 0.91 -10.21

Ps1120d 2153.7 O 1.61 -6.20 Ps1161m 2194.0 M 20/80 -1.31

Ps1122d 2155.4 R 1.38 71/29 -10.10 -9.64 Ps1163s 2195.7 R 0.66 80/20 -10.74 -10.42

Ps1124g 2157.7 R 2.16 -9.20 Ps1164_10 2196.3 O 50/50 -0.82

Ps1126d 2159.7 R 2.61 67/33 -9.25 -8.71 Ps1165s 2197.7 R 0.41 -10.29

Ps1128d 2161.7 R 2.96 -8.87 Ps1167d 2200.1 R 0.31 64/36 -10.00 -9.41

Ps1129_30a 2162.1 D 3.28 57/43 -9.41 -8.72 Ps1168_15 2200.7 M -0.04 75/25 -7.32 -6.92 Ps1129_30b 2162.1 O 2.53 22/78 -6.94 -5.68 Ps1169_15 2201.9 R -1.29 63/37 -11.07 -10.46

Ps1130d 2163.0 D 3.40 -8.57 Ps1169 2202.0 N 0.01 -9.61

Ps1132g 2164.4 R 3.91 -7.78 Ps1172g 2205.0 R 0.00 64/36 -10.83 -10.24

Ps1133_60 2165.7 D 4.02 58/42 -8.44 -7.76 Ps1172_80 2205.5 M 20/80 -1.31

Ps1134s 2166.6 D 4.06 -8.26 Ps1175s 2208.2 R -0.24 69/31 -10.88 -10.38

Ps1135s 2167.6 D 2.19 74/26 -8.10 -7.68 Ps1175_80 2208.5 M 25/75 -1.22

Ps1136_15 2167.9 D 3.41 55/45 -10.59 -9.86 Ps1178_80 2211.5 W 0.79 82/18 -9.08 -8.79

Ps1137_30 2168.7 D 4.12 53/47 -9.33 -8.58 Ps1180_40 2213.1 M 33/67 -1.09

Ps1137d 2169.1 D 2.62 64/36 -11.36 -10.77 Ps1180d 2213.4 R -0.12 69/31 -10.65 -10.15

Ps1138s 2170.1 R 3.72 -10.13 Ps1185s 2218.2 R -0.09 33/67 -11.22 -10.14

Ps1190s 2222.5 R -0.30 -10.63

sample depth facies ¹³C

calcite/

/dolomite¹⁸O corrected ¹⁸O

sample

laminated calcisiltites

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have been used for model calculation. Estimated δ¹³C and δ¹⁸O values for pure calcite and dolomite components were calculated as end-members in the mixing model of two adjacent samples. Due to the obvious inaccuracies of this method, the obtained results are treated as approximate estimations, and used only for estimating the relative influence of dolomite (increasing vs. decreasing values) on the bulk-rock isotope ratios.

RESULTS

Lithology (macrofacies)

The carbonate anomaly of the Reda Member (Text-fig. 2; 2186–2166 m) in the Pasłęk IG-1 borehole section occurs within the thick siliciclastic complex of the distal parts of the Kociewie Formation. In the lower part of the measured interval (2272–2198 m;

Text-figs 2B, 3B), the Kociewie Formation is composed of more or less equal proportions of clay-dominated [fine claystones (J) – 2%; laminated claystones (L) – 27%; mudstones (K) – 16%,] and silt-dominated components [silty mudstones (N) – 26%; calcareous heterolites (R) – 10%; siltstones (P) – 12%; silt laminated mudstones (O) – 3%].

The first change leading to the facies anomaly is the gradual appearance of the specific dolomitic mudstone facies (M; Text-figs 2B, 3B), appearing for the first time (2237 m) 51 m below the main anomaly.

Facies M shows a varve-like compositional lamination, traces of the presence of fine pyrite and towards the top an increasing presence of specific, very thin red clay laminae. Above 2198 m (12 m below the base of the Reda Member), dolomitic mudstones (M) occur more continuously and are the main fine-grained facies component (23% on average; Text-fig. 2B).

Concurrently, in the same interval (2198–2186 m), the participation of siltstone facies distinctly increases (22 to 32% on average), at the expense of other fine- grained facies (J + K + L decreasing from 43 to 19%).

The mass appearance of calcite grains at the base of the Reda Member (Text-figs 2B, 3B; 2186 m) results in the appearance of laminated calcisiltite facies (A–C), which are calcite-bearing textural equivalents of the dolomitic mudstones (M) occurring below.

Facies A (calcisiltites), B (laminated marly calcisil-

tites), and C (laminated calcareous mudstones) were distinguished macroscopically on the basis of the rise in clay content. The facies show a distinct varve-like lamination resulting from compositional variations between laminae and the total absence of bioturba- tion. The rocks appear grey with a red-violet hue due the admixture of some red pigment to the terrigenous laminae. Laminated calcisiltite facies strongly dominate (A: 22%; B: 51%; C: 7%; 80% in total) in the lower (carbonate-dominating) part of the Reda Member (2186–2173 m), with only a minor participation of dolomitic mudstones (M: 9%) and texturally massive calcisiltites (D: 9%).

The termination of the carbonate-bearing anomaly (Text-figs 2B, 3B; 2173–2166 m) consists of numerous, thick (up to 50 cm) massive calcisiltite (D) beds (27%), dolomitic mudstones (M: 17%), reap- pearing laminated claystones (L: 40%) and mudstones (K: 14%). The return of the background facies assemblage (interval: 2166–2123 m) is characterised by a distinctly lower contribution of the silt fraction in comparison to the beds immediately below the Reda Member. The complex is dominated by laminated claystones (L: 45%), and mudstones (K: 13%), with subordinate silt laminated mudstones (O: 19%), calcareous heterolites (R: 12%) and rare intercala- tions of dolomitic mudstones (M: 5%). In the next interval (2123–2050 m), referred by Modliński et al.

(2006) to the Puck Formation, further decline of the silt facies components proceeds (Text-fig. 2B). At a depth of 2050 m (section measured up to 1990 m), a strong facies shift toward the domination of fine claystones facies (J) is observed.

Updated graptolite biostratigraphy

The existing archival biostratigraphic graptolite data (Tomczyk 1973) precede the novel taxonomy and stratigraphy of the upper Ludfordian graptolite fauna introduced by Tsegelnjuk (1976) and Urbanek (1997).

For this reason the graptolite fauna occurring especially above the Reda Member needs to be revised.

The Kociewie Formation below the Reda Member contains abundant specimens of Bohemograptus bo- hemicus (Barrande, 1850) and rare Neocucullograptus spp. up to a depth of 2200.5 m (Text-figs 2C; 3A).

Above this level, the dolomite mudstones – siltstone complex, as well as the Reda Member are almost de-

Text-fig. 4. Thin-section photomicrographs illustrating details of the pre-event facies i.e., immediately below the Lau event interval (below the Reda Member) in the Pasłęk IG-1 core; colour marks on the photographs indicate the mineral composition: red – quartz, green – mica, blue – calcite, orange – dolomite, orange/red outline – abraded dolomite, black – organic matter, yellow – pyrite. A-B – Changes observed in the laminated mudstone facies, between 2218 m (A) and 2187.5 m (B) (respectively 32 and 1.5 m below the base of the Reda Member; uniform magnification);

note the appearance of abundant dolomite silt inside the aggregates (one aggregate is outlined by a white dashed line) toward the top (B); transmit-

→

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Ps1185_20_x10 02 Ps1155_60 10 x10 2016 10 24

Ps1153d x 20 09 compODB

50 µm

Ps1153d 24 comp x20 2016 10 20

Ps1154s 01 nk x10 2016 10 21

50 µm 50 µm

50 µm

A B C

D

E

E’

ted light, one nicol; C – Laminated siltstone facies, 0.5 m below the base of the Reda Member, with visible common sorting of quartz and dolomite grains; note that the dolomite grains not exceed 40 µm in size, whereas quartz grains reach up to 60 µm; the clay bearing laminae in the middle contain only minute quartz and dolomite silt grains; transmitted light, nicols crossed (note unusual interference colours due to atypical thin-section thickness); D-E – Rock-forming elongated aggregates of grains just below the base of the Reda Member (depth 2186 m) in transmitted (D) and combined transmitted and reflected light (E); one nicol (stained with Dickson’s solution). Opaque binder of aggregates is composed of clay and filamentous organic matter (enlarged in insert E’); note that pyrite preferentially occurs outside the grain-bearing core of the aggregates and minute

pyrite grains forming a ‘pyrite-halo’ around the aggregates (yellow marks, opaque in D and bright dots in E).

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void of graptolite macrofossils up to a depth of 2177 m, where sparse fragments of pristiograptids are observed. Above the carbonate- bearing interval, a post-extinction biostratigraphically important graptolite fauna was observed (Text-figs 2; 3A) in a succession typical for the entire Polish Basin (Urbanek 1997) i.e.: Pseudomonoclimacis latilobus (Tsegelnjuk 1976) – FO 2156 m; Slovinograptus balticus (Teller 1966) – FO 2133; Uncinatograptus acer (Tsegelnjuk 1976) – FO 2122 m; and a single point finding of Uncinatograptus acer aculeatus (Tsegelnjuk 1976) – FO 2056 m. The topmost part of the Silurian succession in the analysed section contains graptolites of the Uncinatograptus spineus group (FO 2010 m) indicating its latest Ludfordian age.

Microfacies and mineral composition

The ‘varve-like’ laminated facies observed in the event interval (Text-fig. 3B) shows an evolution cor- responding to general changes in mineral composition (Text-fig. 3H). The gradual increase of dolomite and quartz silt toward the base of the Reda Member is coeval with the appearance of the dolomitic mudstone facies (M; Text-figs 2C, 3). At the base of the Reda Member, the appearance of calcite transforms facies M into laminated calcareous mudstones (C) and calcisiltites (A, B).

The laminated texture of the dolomitic mudstones (M) results from the appearance of horizon- tal accumulations of lens-shaped organic-silt aggre- gates (Text-fig. 4A vs. B). The accumulations show sharp bases and tops, with no signs of reworking or erosive base surfaces. Single aggregates (~1 mm long and ~0.1 mm thick) are composed of dolomite, quartz, and muscovite silt (up to 40 µm in size) and are bonded by organic-clay matter (Text-fig. 4D, E). The dolomite crystals show diverse roundness conditions – from almost euhedral crystals to well- rounded grains (Text-fig. 4E – orange/red marks).

Fine pyrite framboids are most abundant between the aggregate accumulations in clay-mud horizons.

Framboids are sparse in the interiors of aggregates but show local concentrations outside at the aggregate margin, forming a ‘pyrite halo’ (Text-fig. 4D, E).

The subordinate single red-clay laminae consist of fine clayey matrix, with abundant (up to 40%), very fine, individual, distinctly oxidized pyrite framboids;

numerous graptolites and organic cyst cross-sections.

The dolomite silt enrichment, below the base of the Reda Member, is also manifested in the coarse-grained facies. The siltstones below the dolomite-mudstone bearing interval (up to 2198 m) are

dominated by quartz-mica silt and only in some cases contain carbonates, represented by dispersed, fine biodetritus, blocky calcite cement and rare fine detrital grains. Toward the top – in the event prelude, siltstones become dolomite-rich and show common sorting between quartz and dolomite expressed as clay-silt lamination (Text-fig. 4C) and normal grad- ing inside the laminae. The dolomite crystals show different roundness. Additional blocky dolomite cement between the dolomite grains as well as dolomite-cemented aggregates are locally noted in the dolomite abundance maximum, just below the base of the Reda Member.

The first appearance of non-detrital calcite occurs as a component of silt aggregates in the dolomitic mudstone facies (M). Toward the base of the Reda Member, calcite evolves from single anhedral crystals within aggregates in facies M (Text-fig. 4E – blue mark); through elongated, thin overgrowths-ingrowths (Text-fig. 5A, B) and subhedral bulging growths on the aggregate surfaces (as e.g., the spher- oid illustrated in Text-fig. 5C) – in the base of the Reda Member (facies C); to individual calcite rafts (Text-fig. 5D), euhedral grains and thick incrustations of aggregates (Text-fig. 5E) within the Reda Member.

The distinct calcite enrichment (according to XRD data – Text-fig. 3H), starts at the base of the laminated calcisiltite facies interval and causes dilution of detrital components. Concurrently, the dolomite contribution to the detrital material (dolomite normalized to dolomite + quartz + feldspar) attains the maximal level across the ‘sparoid’-bearing interval.

The main, rock-forming components in the calcisiltite facies of the Reda Member are character- istic calcite grains referred as ‘sparoids’, described earlier in detail in the Reda Member of the Mielnik IG-1 section (Kozłowski 2015; no. 14 in Text-fig. 1).

Compared to that case, the calcite grains in the Pasłęk-IG1 section are more complex and occur as successive micromorphologic types. The most primary type (‘cloudy sparoids’) consists of individual euhedral to subhedral equant calcite crystals (10 to 50 µm in size), with a dominant rhombohedral morphology. The crystal centres contain anhedral clouds of filaments (Text-fig. 6A–C, F) in the shape of empty nanotubes (e.g., Text-fig. 6D, F, G). The cloudy centres are overgrown by limpid calcite up to an euhedral crystal shape (Text-fig. 6F; and also A, B, C, G). The outside veneers of the crystals are sometimes unstained (Text-fig. 7C), recognised in SEM-EDS as a Mg-enrichment. The cloudy sparoids, beside individual crystals (as e.g., in Text-fig. 6B, lower part), form crystal clusters or series, developed

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Ps1153d 21comp x20 nk 2016 10 20

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Ps1153d 33 comp x20 2016 10 20

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Ps1153d_x20_14

A

C B

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Ps1148d_x20_07

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Ps1150d 11a x20 2016 10 25 raft

D

E

Text-fig. 5. Thin-section photomicrographs illustrating initial appearance and later development of calcite precipitate growths (blue marks, stained in pink), which evolved in the succession from incrustations or ingrowths of non-carbonate grain aggregates (A-C) at the base of the Reda Member (2185.7 m); to one-side growing individual calcite rafts (D – 2183 m); and incrustations of calcite grain aggregate (E – 2181 m) in the middle part of the member; note sparoid spherulite growing on organic-clay aggregate in C (yellow mark); and transition between calcite bounded ‘rigid’ flake (E – centre) into organic matter bounded aggregate sector with sparoid chain on the right (yellow dashed line); all samples

stained with Dickson’s solution, transmitted light, one nicol.